Silicon carbide (SiC) has a dielectric breakdown electric field that is an order of magnitude larger than that of silicon (Si). Further, compared with silicon (Si), silicon carbide (SiC) also has properties including a band gap that is three times larger, and a thermal conductivity that is about three times higher. As a result, SiC holds considerable promise for applications to power devices, high-frequency devices, and high-temperature operation devices and the like. In recent years, SiC epitaxial wafers are increasingly being used for the above types of semiconductor devices.
SiC epitaxial wafers are produced using SiC single crystal wafers processed from SiC ingots as the substrates for forming SiC epitaxial films. A SiC epitaxial wafer is typically produced by growing a SiC epitaxial film that functions as the active region of a SiC semiconductor device on a SiC single crystal wafer using a chemical vapor deposition (CVD) method.
Further, SiC ingots are obtained by growing SiC single crystal seeds. High-quality SiC ingots having small amounts of defects and different polytypes are desirable, and therefore the SiC single crystal seed used as the growth starting point for the SiC ingot is preferably a single crystal seed for which the levels of defects and different polytypes can be controlled.
In the past, dislocations such as threading screw dislocations, threading edge dislocations and basal plane dislocations tended to exist in SiC single crystal wafers in countless numbers of at least 104/cm2, and eliminating micropipes that became killer defects was a major problem.
However, as a result of recent improvements in technology, wafers having substantially no micropipes and having not more than 104 dislocations/cm2 are now able to be produced. Particularly in the case of screw dislocations, it has been reported (Non-Patent Document 1) that by performing crystal growth using a seed produced using a method such as the repeated a-face (RAF) method, the number of screw dislocations in the wafer can be reduced to not more than 10/cm2. However, another problem arises in that if the screw dislocation density is extremely small, then different polytypes tend to occur.
Generally, the crystal structure of SiC has polytypes including 3C—SiC, 4H—SiC, and 6H—SiC and the like. When viewed from the c-face direction (the <0001> direction), the outermost surface structures of these polytypes are no different. Accordingly, when crystal growth is performed in the c-face direction, the crystal structure can easily change to a different polymorph. In contrast, when these polytypes are viewed from the a-face direction (the <11-20> direction), the outermost surface structures of the crystal structures have differences. Accordingly, in the case of crystal growth in the a-face direction, these polymorphic differences in crystal structure can be inherited. In other words, different polytypes are unlikely to occur.
On the other hand, even in the case of c-face growth, by performing step-flow growth on a growth surface slightly offset from the c-face, the crystal structure can be preserved (and the polymorph differences can be inherited). However, even in those cases where step-flow growth is performed, a-face parallel to the c-face (a c-face facet) is necessarily exposed on the growth surface. This plane that is parallel to the c-face has different growth behavior, and forms a facet growth region. At the stage where the facet growth region is formed, if a screw dislocation exists within the c-face facet, then polymorphic differences in the crystal structure can be inherited during the crystal growth process. In the case of screw growth from a screw dislocation starting point, because the screw portion forms a step, growth in the a-face direction becomes possible, and polymorphic differences in the crystal structure can be inherited. In contrast, when no screw dislocation exists within the facet, then island growth occurs, and polymorphic differences in the crystal structure cannot be inherited. As a result, different polytypes tend to develop within the single crystal. In other words, in order to maintain the polymorphic differences, the facet requires a screw dislocation, and therefore a technique for introducing screw dislocations into a facet is required.
For example, Patent Document 1 discloses a production method for a silicon carbide single crystal that involves growing SiC on a dislocation-controlling seed crystal having a growth surface with an offset angle of not more than 60 degrees from the {0001} plane, and having a screw dislocation formable region on the growth surface.
By growing SiC on the dislocation-controlling seed crystal having a screw dislocation formable region described in Patent Document 1, screw dislocations can be formed reliably within the c-face facet, meaning the occurrence of different polytypes and different orientation crystals can be suppressed.
Further, Patent Document 2 discloses that by ensuring that the growth end face of the seed crystal has a convex shape with a prescribed curvature, a c-face facet region is formed that enables spiral growth during the growth process.